Diffusion-controlled processes in porous structures are analyzed using planar stochastic fractal objects, which are related to known preparation procedures of porous materials. The fractal objects are cluster-cluster aggregates, the "negative" of cluster-cluster aggregates (i.e. pore fractal), diffusion-limited and reaction-limited aggregates. Random clusters are also constructed for comparison purposes. The processes considered in this study are diffusion driven by an externally imposed concentration gradient and diffusion driven by reaction. The results are presented in the form of concentration contours, effective tortuosities and reaction effectiveness factors. These classes of fractal objects differ in their effective properties (summarized by a tortuosity-porosity relation) and differ from the behavior of fully random objects. This work suggests that tortuosity predictions should be classified according to the manufacturing method of the catalyst.